1. Field of the Invention
The present invention relates generally to vehicle-mounted aerial devices, and more particularly to structural components of such devices which are constructed from dielectric composite materials.
2. Background and Prior Art
Vehicle mounted aerial devices have long been used for a variety of applications such as performing work on utility poles, trimming trees, maintaining street lights, and servicing overhead power and telephone lines. The aerial device normally includes a multiple-section boom which can either be an articulating boom or a boom that is extensible and retractable in telescoping fashion. The end of the upper boom is equipped with a personnel carrying device which is typically a platform, sometimes called a “bucket.” The aerial work platform assembly consists of: the mounting brackets, platform, jib, the control assembly, control input mechanism and all other components at the end of the upper boom. This assembly is commonly referred to as the “boom tip” More than one platform may be attached to the end of the upper boom, and a platform may be large enough to carry one or more workers. Supplemental load lifting devices may also be installed on the boom near the platform in order to provide the aerial device with material lifting capabilities, in addition to its personnel lifting feature. The load lifting device is typically an adjustable jib, a winch, or a combination of both.
Typically, an aerial device broadly comprises a platform which serves as a work station for the operator; a movable boom; a vehicular base, such as a truck; a control input mechanism; and a control assembly. The platform is operable to lift or otherwise carry at least one worker to the elevated work site, and is coupled with the boom at or near a distal end thereof. Because the platform may be used near highly-charged electrical lines or devices, the platform is typically electrically isolated from the ground through the insulated booms and vehicle base so as to provide secondary protection against damaging electrical discharge or electrocution of the worker or bystanders. One component in isolating the platform occupant from ground through the booms and vehicular base is a non-conductive platform liner which provides some electrical isolation for the occupants lower extremities, as long as the lower extremities are contained entirely within the liner and in contact with nothing other than the liner.
The booms are movable so as to elevate and otherwise position the platform where desired, and are coupled with the vehicular base at or near a base end of the lower boom which is substantially opposite the distal end. The upper boom is constructed of an electrically non-conductive, or dielectric, material and provides secondary protection by preventing a path to ground through the booms and vehicular base. Commonly, in order to further electrically isolate the platform from electrical discharge via the boom and the vehicular base, an intermediate portion or section of the lower boom is constructed of or covered with an electrically non-conductive, or dielectric, material. The distal end of the boom or boom tip however, though electrically isolated from the vehicular platform, must incorporate structural material so as to have sufficient structural strength to support the platform and worker. This structural material is typically an electrically conductive metal, such as steel, with the steel, platform and control assembly being considered electrically connected. In addition to the boom assembly, various other parts at the end of boom are constructed from metals such as steel or aluminum and all components at the end of the boom must be considered electrically connected. The vehicular base is motorized and wheeled or otherwise adapted to quickly and efficiently travel to and from the work site. The vehicular base will either be in direct contact with an electrical ground, such as, for example, the Earth, or must be considered in direct or indirect contact therewith.
The control input mechanism allows the elevated worker to provide a control input to control, via the control assembly, movement of the boom and positioning of the platform. Commonly, the control assembly comprises one or more hydraulic control valves, one or more fluid conduits and a quantity of hydraulic fluid, to transmit the control input down the boom for implementation. The necessary conduit connections, however, prevent the control valves from being located inside the platform and its protective liner. Furthermore, as the control input mechanism must be in direct physical contact with the control assembly in order to actuate the valves in accordance with the control input, the control input mechanism without proper protective equipment must also be located outside the platform and protective liner. Thus, the worker may reach outside the protective liner to actuate the control input mechanism, thereby exposing him or herself to possible electrocution if they are working in the area of energized lines, contrary to federal safety regulations and employer safe practices. The control valves to which the control input mechanism is coupled are typically constructed of an electrically conductive material. Furthermore, the control valves may be located in close proximity to the aforementioned electrically conductive structural support material used to reinforce the distal end of the boom.
Thus, although the aforementioned dielectric boom portion does protect against electrical discharge via the boom and vehicular base, it does not protect against direct discharge via the electrically conductive structural material in the distal end of the boom, via the control valves, and via the control input mechanism. For example, a discharge path could be from an unprotected first conductor, to any component at the boom tip, to any other component at the boom tip, including the control input mechanism, to a worker not using rubber gloves, and to a second unprotected conductor. It will be appreciated that the dielectric boom portion provides no protection against this or similar discharge paths.
In order to minimize the risks of injury, the operator must always maintain safe clearances from electrical lines in accordance with applicable government regulations, such as those promulgated by the Occupational Safety and Health Agency (OSHA), and safe work practices adopted by the employer. Furthermore, if the possibility of electrical contact or proximity exists, operators must use proper protective equipment which provides primary protection from electrical injury. The aerial device will not provide protection from contact with or in proximity to an electrically charged power line when the operator or the components at the boom tip are in contact with or in proximity to another power line, ground, or pole. If such contact or proximity occurs, all components at the boom tip, including the controls, may become energized. It should be understood that no invention will completely prevent electrical accidents. However, the present invention provides greater protection than existing designs against electrical injury that may be sustained by a worker whose behavior does not conform to government regulations and safe work practices.
Therefore due to advances in technology and newly available materials, an opportunity now exists for an improved aerial work platform assembly that may better protect the worker against electrical discharge when regulations and safe practices are not followed. While various non-metals, such as rubber, plastic, and polymer materials might satisfy the dielectric requirement of the components in such an improved system, most of those materials are not suitable. The aerial work platform assembly components must be structurally rigid and durable, but cannot be overly bulky and cumbersome to manipulate. Thus, there remains a need for an aerial work platform assembly that maximizes the number of parts which are lightweight, structurally rigid, durable, and substantially nonconductive, in addition to being more cost effective than the construction of prior art assemblies.